1. Field of the Invention
This invention relates to a light-emitting diode, more particularly to a surface plasmon enhanced light-emitting diode by virtue of coupling with surface plasmons.
2. Description of the Related Art
It is well known that a light-emitting diode (LED) can have an enhanced light-emitting efficiency by coupling surface plasmons with quantum wells in a light-emitting layer (having a multiple quantum well structure) of the LED. The surface plasmons are coherent electron oscillations excited on the metal surface by an electromagnetic wave of light and propagate along a direction parallel to the metal surface in a form of surface electromagnetic wave, the electromagnetic field strength of which is reduced exponentially. The surface electromagnetic waves of the surface plasmons are provided with properties similar to those of evanescent waves and belong to one kind of plane waves.
In general, there are two kinds of metallic structures capable of being excited by and coupled with an incident electromagnetic wave to induce the surface plasmons. One structure is a metal layer formed with a periodically microstructure. The other structure is a metal layer having a material with a relatively high dielectric constant formed thereon. Two conventional light-emitting diodes, both of which are formed with nano-scale metallic structure to induce surface plasmons, are described in the following.
A surface plasmon enhanced LED has been proposed by Min-Ki Kwon et al., “Surface-plasmon-enhanced Light-emitting diodes,” Advanced materials (2008), vol. 20, pages 1253-1257. In the surface plasmon enhanced LED, an Ag nanoparticle layer is inserted between an n-GaN layer and a light-emitting layer of a multiple quantum well (MQW) structure to induce surface plasmons therebetween. However, since epitaxial layers, such as a light-emitting layer and a p-GaN layer, are formed after the Ag nanoparticle layer is formed on the n-GaN layer (also an epitaxial layer), they are not sequentially formed in a continuous epitaxial process on the n-GaN layer but are formed on a heterogeneous material of Ag. Furthermore, in practice, the quantity of Ag nanoparticles in the Ag nanoparticle layer is insufficient to form a continuous layer. Accordingly, the Ag nanoparticle layer is uneven, and the quality of the epitaxial layers that are formed above the heterogeneous material (Ag) is relatively poor. Thus, in this case, it is likely to adversely affect the light-emitting efficiency of the LED.
Another surface plasmon enhanced LED has been proposed by Dong-Ming Yeh et al., “Localized Surface Plasmon-induced Emission Enhancement of a Green Light-emitting diode,” Nanotechnology 19 (2008), p. 345201. The green LED is formed with an Ag layer with Ag nano-island structures on a p-GaN layer in a relatively high density. However, an effective area on the p-GaN layer for light emitting from a light-emitting layer is blocked and limited by the high density of the Ag nano-island structures. On the other hand, in order to ensure that the surface plasmons induced by the Ag nano-island structures can couple with quantum wells in the MQW structure of the light-emitting layer, a thickness of the p-GaN layer, i.e., a distance between the light-emitting layer and the Ag nano-island structures, is limited (e.g., 60 nm), in consideration of the skin depth of the surface plasmons. However, for providing a p-n junction of the LED in an effective operation, it is necessary to maintain a sufficient width of a depletion region in the LED. In this case, the p-GaN layer of 60 nm is insufficient for maintaining the sufficient width of the depletion region required for a well-operated LED.